Inorganic fine particles are excellent in such as strength and heat resistance, and therefore, used in various fields. Particularly, silica has been widely used for an anti-blocking agent and a slip agent for various kinds of films such as polyester films, polyimide films, and fluororesin films; a spacer such as an in-plane spacer for liquid crystal display devices, a seal spacer for liquid crystal display devices, a spacer for EL display devices, a spacer for touch panels, and a spacer such as a gap-retaining agent for various kinds of substrates of ceramics, plastics, or the like; a sealing material for various kinds of electronic parts such as a sealing material for semiconductors, a sealing material for liquid crystals, and a sealing material for LED emitting devices; a light diffusion agent to be used for light diffusion films, light diffusion plates, light-leading plates, anti-glare films, or the like; an additive for cosmetics such as white body pigments; a dental material; and the like.
Such silica has been conventionally produced by hydrolytic polycondensation of a silicon alkoxide (alkoxysilane) (Patent Documents 1 to 3).
The applicant of the present application also employs a hydrolytic polycondensation process of a silicon alkoxide at the time of producing fired silica with low hygroscopicity (Patent Documents 4, 5, and 6). In Patent Document 5, particularly, it is proposed to employ a vacuum drying apparatus in order to obtain a dry powder of silica particles without agglomeration after having got a slurry of silica particles in monodispersion state. This vacuum drying apparatus is used for drying a liquid containing solid content in reduced pressure while heating the inside of a long tubular heater to introduce the obtained powder into a vacuum collecting chamber, followed by collecting the powder by filter, and is configured not to cause secondary agglomeration since the particles in the slurry are dried almost instantaneously.